A forced air furnace forces heated air into a home using a circulation fan which delivers air over the furnace's heat exchanger and into the duct distribution system. The air is then returned to the furnace through intake vents for re-circulation through the heating system. In order for a forced air furnace to run most efficiently, the air delivery of the heating system should remain relatively constant at a certain fixed value of cubic feet of air per minute. The air delivery of a heating system is a function of the air pressure produced by the circulation fan and air delivery restrictions in the heating system. The static pressure present within a heating system is indicative of the air delivery for a fixed circulation fan speed. Static pressure is the steady state pressure that exists within a system for a fixed fan speed and is commonly measured in units of inches of water.
Typically, installers of forced air furnaces are responsible for determining and implementing the correct fan speed for each installation. Static pressure and other heating characteristics must be measured to determine an efficient air delivery rate for the particular air duct restrictions and characteristics of a heating system. After a forced air furnace is installed, further changes in air delivery restrictions requires further air delivery speed adjustments. However, air delivery installation testing and adjusting is rarely done in practice and post-installation air delivery adjustments are not likely to be made by the dwelling occupants.
Air delivery restrictions can be caused by duct blockages such as dirty air filters, dust and dirt build up, and other restrictions in the vents. Factors relating to the specific configuration of the vent system also affect air delivery, such as the width and length of the ducts used and the number of elbows in a duct passage. The opening and closing of individual warm air registers or cold air return vents also significantly affect the air delivery rate of a given installation. The presence of air delivery resistance produces a decrease in the air delivery of a furnace and reduces heating system efficiency.
Furnace efficiency is related to a balanced air delivery at a particular heat rise. Heat rise is the difference between the temperature of the warm air being produced by the furnace and that of the cold intake air. For efficient furnace operation, it is known that the heat rise should remain constant at a value of approximately 70.degree. F. When air delivery restrictions are present in a heating system, the rate of air delivery is reduced and heat rise is increased. Furnace efficiency is decreased due a slower stream of air passing through the heat exchanger at a comparable temperature to that of the heat exchanger. This results in a significant amount of heat not being transferred from the heat exchanger to the air being delivered over the heat exchanger. This heat is then lost through the combustion flue. This inefficiency also results in hotter vented combustion products and may present problems for plastic vent materials.
One solution is to install a manual fan speed control device which allows a home owner to manually adjust circulation fan speed. However, these systems are commonly set and left for long periods of time at high speed settings in order that as much heat as possible is efficiently extracted from the heat exchanger. Air moving at higher velocities results in the cooling of human skin due to increased evaporation of moisture on the skin's surface and causes discomfort to the occupants. In addition, increased air velocities result in increased noise within the building. While this solution is relatively inexpensive, it is inefficient and unreliable as a long term solution as such manual adjustments can be made in error or not at all due to the device's inability to automatically adapt to changing air delivery resistances.
Other fan speed control systems control circulation fan speed to delay the execution of safety shut-down procedures when the system reaches dangerous operating levels. For example, U.S. Pat. Nos. 4,705,881 and 4,792,089 to Ballard, both disclose a furnace control system which increases the speed of an air blower by alternately engaging higher motor speed windings when the temperature of air to be heated exceeds a pre-determined temperature. When high-limit conditions are detected, the control system advances the speed of the circulation fan in association with higher motor windings, typically over two or three motor speeds. The controller stops increasing fan speed if the temperature drops below the pre-determined temperature. However, if the top fan speed is reached and the temperature remains above the predetermined temperature then shut down procedures are initiated. While this control system varies the circulation fan speed in response to detected air delivery resistance, it does not allow the circulation fan speed to be adaptively increased or decreased during the normal course of operation in response to varying air delivery resistances.
More sophisticated attempts to address changes in air delivery due to air delivery restrictions have involved attempts to control the fan motor speed in response to changes in motor load characteristics during normal operating conditions. For example, U.S. Pat. No. 5,524,556 to Rowlette et al. discloses a fan motor controller which detects changes in parameters such as motor torque and motor speed and makes corrections to the fan motor to maintain constant air delivery despite changes in air delivery resistances. Corrections are made using a microprocessor which reads motor speed and torque and then computes desired speed based on a torque-speed characteristic stored in memory. However, such reactive control techniques typically result in fan speed changes of more than 15% which causes undesirable wind chill effects. Thus, while circulation fan speed is being adjusted during the course of normal operation, this solution is only partially effective due to its crudely reactive nature and associated construction and installation costs.
Accordingly, there is a long-standing need to improve the efficiency of forced air furnaces, to improve the level of occupant comfort, and to eliminate the need for air delivery calibration as part of the furnace installation procedure, using a control system which provides a highly adaptive response to changes in air delivery and which is relatively inexpensive to manufacture and install.